This invention relates in general to elastomeric bearings and, in particular, to a laminated bearing for supporting the rotor blade of a helicopter.
More specifically, this invention relates to an improved elastomeric bearing assembly of a laminated construction employing the combined effects of a spherical retention bearing and a pair of serially arranged frusto-conical thrust sections to improve the method of retention of the rotor blade of a helicopter. The laminated elastomeric bearing assembly of the invention comprises an assembly having minimal size and weight and, an effective increase of bearing life.
A helicopter rotor blade undergoes a multitude of complex planar and non-planar movements during rotation to develop lift and induce directional movement of the aircraft. The cross section of a helicopter blade possesses a configuration similar to the air foil of a fixed-wing airplane. Upward lift to overcome the weight of the helicopter is achieved by the passage of the air-foil shaped blade through the air. This lift is controlled by selective manipulation of the angular orientation of the individual blades while being swept through the air with a rotary motion. Blade orientation is further varied to effect movement of the helicopter in any direction, decrease its altitude, or maintain the craft stationary while airborne. Such variation in orientation involves alterations of angle of attack during each revolution of the rotor to produce oscillating pitch changes of the blade through rotation about its longitudinal axis. In addition to pitch changes, the rotor blade undergoes movements in the plane of rotation, known in the art as lead/lag, and oscillatory movements substantially perpendicular to the plane of rotation, commonly referred to as flapping.
Because of the foregoing need for constant adjustments of the helicopter rotor during flight, its blade support system must accommodate a multitude of motions and must resist a complex pattern of forces without failure. The mere rotation of a plurality of rotor blades imposes large centrifugal forces on the blade bearing support. Moreover, oscillatory changes about the axis of the blade to alter pitch subjects the blade retention system to a considerable torsional loading. In addition to the centrifugal and torsional loads, a blade retention system must also resist forces arising in conjunction with such factors as lead/lag, and flapping movements.
The design of a helicopter necessarily requires the application of sophisticated technology, in part because of the foregoing dynamics and force patterns. The retention system of a rotor blade plays a vital role in the operation of the helicopter and must accommodate the multi-directional forces which are encountered while accommodating the required blade motions. Frequent and continual failure of the bearing support of a blade is costly and potentially detrimental to the structural integrity of the helicopter, particularly if problems develop in the bearing assembly while the aircraft is in flight. Simple economics and efficiency demand, however, that lengthy operative service of the bearing be attained by a lightweight assembly of minimum cost. It is a continuing problem in the prior art that known bearing supports of rotor blades require components of undesirable high weight or employ exotic and expensive materials to attain suitable service.
In the prior art, the foregoing and other forces, inherent in the rotation of the rotor of a helicopter, were supported by means of non-friction bearings in earlier versions of helicopters. Non-friction bearings proved unsatisfactory because of a short lifetime of service, a relative heavy weight, and the requirement of frequent servicing among many problems. The shortcomings of non-friction bearings were, in part, overcome by the introduction of laminated elastomeric bearings of various designs. Some of these laminated support assemblies employ different combinations of bearing sections having alternate layers of elastomeric material bonded to a more rigid shell material, such as metal.
In an elastomeric rotor support system it is common practice to use an elastomeric bearing with spherical laminations to accommodate the three angular displacements required of the blade while sustaining all blade reaction forces. It is difficult to design a spherical bearing that will sustain the full amount of the pitch change motion required. Frequently an additional elastomeric bearing is added to the system to handle most of the pitch change motion while carrying only the centrifugal load. At first thought it would seen that the laminates for this bearing should be flat annular washers. For practical designs this idealistic bearing seldom has column stability. The prior art solution to this stability problem has been to step the shells or convolute them as shown in Rybicki, U.S. Pat. No. 4,142,833. The results have always been a large increase in stress and strain over the idealistic flat laminated bearing.
The laminated constructed prior art blade support bearings have also required expensive material and costly manufacturing techniques in order to achieve a satisfactory lifetime of service and high resistance to failure. The type of elastomeric bearing as shown in U.S. Pat. No. 4,142,833 to Rybicki employs an annular thrust bearing of a laminated construction to absorb centrifugal forces generated by the rotation of the blades. The convoluted metal laminates in the annular section of Rybicki are subject to high stress in compression because the resultant force vectors applied to the thrust bearings are not perpendicular to the planar face of the laminates. Accordingly, the metal laminates must be made of high strength material to resist the resulting bending and avoid fatigue and early failure. The elastomer layers must also be more closely spaced to resist the non-uniform strain found in this type of bearing.
Another prior art bearing system is shown in U.S. Pat. No. 4,028,002 to Finney, et al. and relies upon the combined effect of spherical and conical laminated bearing sections to retain the blade during its multitude of movements. The conical sections of the bearing of Finney, et al. are intended to provide great lateral stiffness with the result that longitudinal loads produce very high elastomer strains. Such a technique also does not achieve optimum support of a rotor blade of a helicopter.